The present invention relates to a system for detecting a crank angle relative to a specific cylinder of an engine for a motor vehicle, and more particularly to a system for discriminating a cylinder to be ignited.
Japanese Patent Application Laid-Open No. 57-124208 discloses a cylinder discriminating system in which a disk is provided to rotate once while a crankshaft of an engine rotates twice and to generate a crank angle signal in the form of pulses sensed by a sensing means. The crank angle signal is used for designating a cylinder within a discriminating range on the disk.
FIGS. 5a and 5b show a diagram in relation to the crank angle signal and a disk 52 secured to a crankshaft in a conventional system. The system has a cam shaft disk having projections and produces cam signal pulses CS for every cylinders. The crank angle signal CA in the form of pulses is generated at projections formed on the periphery of the disk 52 in an angular discriminating range of the crankshaft and the number of the pulses CS is counted to discriminate a corresponding cylinder. The angular discriminating range is shown by C in FIG. 5b, which is between 10.degree. and 100.degree. before the top dead center (BTDC) for the cylinder.
On the other hand, in a four-cylinder engine as an example, there is high probability that the engine stops near an angle of 70.degree. before the top dead center on the compression stroke of a cylinder, which is included in the discriminating range C. Reference S in FIG. 5a designates an engine stop range in which the engine frequently stops. Accordingly, if the engine starts under the condition that the engine has stopped in the range S, the cam signal pulses may be counted in error. In other words, the counted number of the pulses may become less than the actual number thereof, because the pulses CS may be included in the stop range S. Thus, accurate ignition timing is not obtained, which causes backfire of the engine.